The present invention relates to a method of manufacturing an integrated circuit ("IC") hybrid balance network for use with a subscriber line interface circuit of a telephone system, and more particularly to a method of manufacturing a IC hybrid balance network with a fixed number of selectable network configurations, where the number of possible configurations of the manufactured network is increased by the manufacturing process.
A telephone system subscriber line interface circuit ("SLIC") connects a balanced two-wire transmission path (the path to and from the subscriber telephone handset) with an unbalanced four-wire transmission path (the path to and from the telephone central station). SLICs perform various functions, including battery feed, overvoltage protection, ringing, signaling, hybrid balancing, and timing.
The four-wire transmission path between the SLIC and central station may employ either analog or digital signals. A digital signal transmission being created by the transmit analog section of a Codec and the digital signal received being converted to an analog signal by the receive analog section of the Codec. A hybrid balance network, placed intermediate to the receive analog section and transmit analog sections of the Codec and connected to the SLIC, cancels any echo between the two Codec sections.
A hybrid balance network typically consists of three sections: a high pass filter, a low pass or equalizer filter, and a gain section. Networks may be uniquely designed to provide a single configuration of filter capacitors and gain controlling amplifier resistors for an identified SLIC. However, the number of network configurations possible for a uniquely identified SLIC may be increased without changing the masks used in the manufacturing process by making the networks programmable, i.e., by selectively switching the fixed number of network elements into and out of the network.
To increase the number of possible network configurations without the need for a new mask in the manufacturing of that network, it is known to include a layer of read only memory, or ROM, intermediate to the switches associated with each circuit element and the underlying circuits of the IC which indicate the desired network configuration. In this way, the ROMs may be blown/programmed in accordance with a particular configuration program in the manufacturing process to thereby determine which of the fixed number of switches is responsive to which signal from the underlying circuit of the IC. While the fixed number of configurations is not increased, the configurations themselves may be changed by this programming in the manufacturing process, and no change in the IC mask is required. Once manufactured, both the number of configurations (determined by the number of switches) and the configuration itself (determined by the interconnection of the switches to the network elements) cannot be changed.
Each of the network sections is configured specifically for the SLIC for which the network will be connected, and the network manufactured for one SLIC will not generally be useful in a different SLIC notwithstanding the fact that the network provides the same fixed number of configurations, i.e., the configurations themselves will be different. Thus new semiconductor masks are required to manufacture a network for each SLIC.
The present invention does not increase the number of configurations available in the manufactured network. However, and in accordance with the present invention, an increased number of possible network configurations are available without the necessity of new semiconductor masks in the manufacturing process. Thus the same semiconductor masks can be used in the manufacture of networks appropriate for a variety of SLICs.
Accordingly, it is an object of the present invention to provide a novel method of manufacturing an IC hybrid balance network.
It is still another object of the present invention to provide a novel method of manufacturing IC hybrid balance networks where the number of possible network configurations selectable during manufacturing is increased without increasing the number of available configurations in the manufactured network.
It is yet still another object of the present invention to provide a novel method of manufacturing IC networks whereby the network configurations may be easily changed without altering the manufacturing process.
It is a further object of the present invention to provide a novel method of manufacturing a programmable impedance element within an IC whereby the possible configurations of the element exceeds the number of controlled switch elements.
In one aspect, one or more ROM matrices may be used with the programming accomplished during or subsequent to the manufacturing process.
It is thus another object of the present invention to provide a novel method of manufacturing plural IC hybrid balance networks using the same masks.
In another aspect, the prior art manufacturing processes for determining which switch was connected to which circuit element, or more specifically where the gain controlling resistor was contacted, used a semiconductor mask to control the configuration of the switches and a metal mask to perform the contacting. The change in the semiconductor mask thus determined the ultimate selection of resistor contact point.
In the present invention, the semiconductor mask is unchanged and the metal mask is changed to thus determine the ultimate resistor contact point. Substantial advantages are obtained by the accomplishment of the programming in the metal mask rather than the semiconductor mask.
Additionally, the number of possible available contact points on the resistor may easily be increased in the mask which defines the resistor, so that far greater flexibility is achieved.
It is accordingly an object of the present invention to provide a novel manufacturing process for hybrid networks in which contacting of switches to circuit elements is controlled by changes in the metal masks rather than changes in the semiconductor masks.
It is thus another object of the present invention to provide a novel manufacturing process for hybrid networks in which the number of possible configurations can be increased without the necessity for changing any semiconductor masks.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.